Clashes between anti-government fighters and regime forces killed 51 combatants on both sides in north-western Syria Tuesday (August 27th), AFP reported.
Russia-backed regime fighters have for weeks been chipping away at the edges of the extremist-run stronghold of Idlib -- a province that borders Turkey -- after bombarding it for months.
But hardline opposition fighters and extremists on Tuesday attacked loyalist positions in the south of the bastion, the Syrian Observatory for Human Rights said.
"Violent clashes east of the town of Khan Sheikhun broke out at dawn after extremists and opposition groups attacked regime positions," Observatory chief Rami Abdel Rahman said.
The attack was led by the al-Qaeda-linked Hurras al-Deen group and another extremist faction, Ansar al-Deen, he said.
The fighting has killed 23 regime forces and 20 opponents, including 13 extremists, the Observatory said.
In the south-east of the bastion, eight opposition fighters were killed trying to sneak through frontlines towards regime positions near the Abu Duhur military airport, the monitor added.
Regime forces recaptured Khan Sheikhun last week, and have been massing north of the town in recent days as they prepare to push on with their assault.
The town lies on a key highway running through Idlib province, and fully recapturing the artery would allow the government to reconnect Damascus to Aleppo.
Heavy regime and Russian bombardment has hit areas north of Khan Sheikhun in recent days, in the vicinity of the town of Maaret al-Numan, the next stop north on the highway.
On Monday, regime and Russian airstrikes killed 12 civilians in the extremist stronghold, the Observatory reported.
The offensive comes despite a deal signed in September last year by Moscow and opposition backer Ankara to avert a full-blown assault on Idlib.
The presidents of both countries were set to meet in Moscow on Tuesday.